Some solid state lasers such as titanium sapphire lasers are attractive in many applications because they are broadly tunable, as compared to other solid state and gas lasers which tend to have more discrete, narrow, line outputs. Titanium sapphire lasers are also desirable because they have high gain and as solid state devices are also durable and stable. Although these lasers are broadly tunable, the gain varies greatly with wavelength and promoting lasing at the lower gain wavelengths can be difficult. Tuning such a laser to overcome the high gain at center wavelengths and still be low-loss at a desired wavelength may otherwise compromise laser performance. Titanium sapphire lasers also have a very short upper state or fluorescence lifetime (approximately 3.2 .mu./sec) which requires very short pulse pumping excitation that approaches the maximum power dissipation capability of the crystal. The risk of loss and damage from the high power can be mitigated by using Brewster-angle cuts on the crystal. Brewster-angle cuts are minimally reflective (highly transparent) to "P" field polarized light but highly reflective to "S" polarization. Thus Brewster-cut crystals provide high transmissivity (low reflectivity loss) without the need for antireflection coatings which are very vulnerable to the high power, short duration pumping pulses. However, the use of Brewster-angle surfaces introduces refractive dispersion problems in the cavity: different wavelengths are directed along different paths.
The desired spectrally narrow or pure (single frequency) laser output can be more nearly achieved with titanium sapphire lasers by use of a seed input in addition to the pumping input. But this has shortcomings. Seed input can be introduced wherever there is laser energy escaping. For example, seed input can be supplied at the output but this requires a Faraday isolator which is complex and puts at risk the seed source. The seed input could also be introduced at the other end of the cavity at the high-reflectivity surface. But this is a problem when that surface must be moved about in order to tune the laser to a desired wavelength and so the seed input must be moved with it. Another problem occurs with titanium sapphire crystals when the pumping energy required approaches the upper limit of the crystal capacity, which can vary from 5 or 10 Joules to 50 Joules or more per square centimeter at the crystal face, depending upon the crystal quality and purity. Not only does high pumping energy put the crystal structure at risk but it induces parasitic laser action in the crystal.
Separately, it is well understood that stable laser oscillators cannot provide more than tens of millijoules of diffraction-limited output before reaching a practical limit due to optical damage. This is so because the TEM.sub.oo mode diameter for stable oscillators is limited to a few millimeters, which leads to high peak power densities on intracavity components at both pump and laser wavelengths. That is, they do not have energy scalability. Unstable laser oscillators, in contrast, provide a simple, compact means of increasing the output energy of diffraction limited oscillation. But conventional unstable oscillators typically provide an annular, or doughnut-shaped output with severe diffraction at the center.